The increasing need to form planar surfaces in semiconductor device fabrication has led to the development of process technology known as chemical-mechanical-polishing (CMP). In the CMP process, semiconductor substrates are rotated against a polishing pad in the presence of an abrasive slurry. Most commonly, the layer to be planarized is an electrically insulating layer overlying active circuit devices. As the substrate is rotated against the polishing pad, the abrasive force grinds away the surface of the insulating layer. Additionally, chemical compounds within the slurry undergo a chemical reaction with the components of the insulating layer to enhance the rate of removal. By carefully selecting the chemical components of the slurry, the polishing process can be made more selective to one type of material than another. For example, in the presence of potassium hydroxide, silicon dioxide is removed at a faster rate than boron nitride. The ability to control the selectivity of a CMP process has led to its increased use in the fabrication of complex integrated circuits.
A common requirement of all CMP processes is that the substrate be uniformly polished. In the case of polishing a electrically insulating layer, it is desirable to polish the layer uniformly from edge to edge on the substrate. To ensure that a planar surface is obtained, the electrically insulating layer must be uniformly removed. Uniform polishing can be difficult because, typically, there is a strong dependence of the polish removal rate on localized variations in the surface topography of the substrate. For example, in substrate areas having a high degree of surface variation, such as areas having closely spaced active devices, the polishing rate is higher than in areas lacking a high degree of surface contrast. Additionally, the polishing rate at the center of substrate may differ from the polishing rate at the edge of the substrate.
To compensate for the varying removal rates at different locations on the substrate surface, the polishing process is extended to ensure that a planar surface is obtained. A hard, thin-film, referred to as a polish-stop layer, can be used to prevent the unwanted removal of material in the underlying device layers during extended polishing. If the polish-stop material is sufficiently resistant to abrasive removal, and the polishing slurry is selective to the polish-stop material, the polishing time can be extended until the passivation layer is uniformly polished, without damaging underlying layers. To be selective to the polish-stop layer, the chemical components in the slurry must be substantially unreactive with the polish-stop material. Common polish-stop materials include silicon nitride and boron nitride, and the like. In the absence of a polish-stop layer, over-polishing can occur resulting in unwanted removal of underlying layers.
To ensure that uniform polishing action is obtained, it is important that the rate of material removal remain constant. Changes in the surface texture of the polishing pad during the polishing process reduce the degree of abrasiveness of the polishing pad. In particular, during the polishing of an insulating material, such as silicon dioxide, reaction products generated in the polishing slurry, and other debris, collect on the surface of the polishing pad. The collected material fills micro-pores in the surface of the polishing pad, which is known as glazing. When the micro-pores become filled with residue from the polishing process, the polishing rate declines. In extreme cases, a decline in polish removal rate can result in an incomplete removal of material leading to a degradation in polishing uniformity. This is because the polishing process is controlled by specifying a time interval for completion of the polishing process. The time interval is calculated based upon a specific and constant polish removal rate.
In order to avoid degradation in the polish removal rate caused by glazing the surface of the polishing pad, the pad is abraded by a conditioner, such as a steel brush. In the abrasion process, material is removed from the surface of the pad by a mechanical grinding process. This process results in removing material from the pad itself in addition to reaction products and debris from the polishing process. Changes in the surface structure of the polishing pad can result in process instability and reduced usable lifetime of the polishing pad.
While CMP potentially offers wide versatility, and the ability to form surfaces with a high degree of planarity, the polishing process must be carefully controlled to maintain optimum process performance. To date, methods to improve processing performance have included the development of high selectivity polishing slurries, and the development of various materials for use as polish-stop layers. However, further development is necessary to provide process parameter stability, and to provide real-time process control for the polishing apparatus.